The present invention relates to a method for the preparation of an aqueous emulsion of poly(silsesquioxane) or, more particularly, to a method for the preparation of an aqueous emulsion of poly(silsesquioxane) having high stability without the disadvantage of formation of gelled particles by use of a cationic surface active agent.
As is well known, aqueous emulsions of a diorganopolysiloxane such as dimethylpolysiloxanes can be prepared by the polymerization in an aqueous emulsion and such aqueous emulsions of silicone are used widely in various applications. A so-called poly(silsesquioxane), which is a type of organopolysiloxanes represented by the average unit formula of RSiO.sub.1.5, in which R is a monovalent hydrocarbon group, can also be obtained in the form of an aqueous emulsion and various methods have been proposed for such an aqueous emulsion of poly(silsesquioxane) although no quite satisfactory method has yet been disclosed. For example, the method successful for the preparation of an aqueous emulsion of dimethylpolysiloxane by use of a cationic surface active agent is not applicable in this case due to the difference in the reactivity of the starting organosilanes and the molecular structure of the resultant organopolysiloxane leading to the formation of a large amount of coarse gelled particles.
Japanese Patent Publication No. 52-12219 teaches a method for the preparation of an aqueous emulsion of a poly(silsesquioxane) in which the starting organosilane compound is added dropwise into an aqueous solution of a surface active agent and a catalyst to give the poly(silsesquioxane) in a colloidal dispersion. This method is disadvantageous because the process must be performed using a special apparatus for the dropwise addition of the silane compound and the dropwise addition of the silane compound must be carried out gradually under well-controlled conditions over an unduly long time. When a cationic surface active agent is used in this method, in particular, formation of a considerably large amount of coarse gelled particles is also unavoidable to cause troubles in filtration by filling up the meshes of the filter cloth or decrease in the content of the silicone in the filtered emulsion if it can be obtained.
Japanese Patent Publication No. 60-20959 teaches a method for the preparation of an aqueous emulsion of a low-molecular liquid silicone resin. The disclosure therein, however, relates to a method of mere emulsifying dispersion of the silicone resin to give an aqueous emulsion applicable as a hydrophobic treating agent for building materials. As is understood from the formulation disclosed there that the emulsion is used with admixture of a curing catalyst of the silicone, the dispersed phase of the emulsion is a low-molecular silicone resin having curability by means of the residual alkoxy and/or silanol groups.
On the contrary, the poly(silsesquioxane) is a fully condensed solid resin insoluble in any organic solvent and, when an aqueous emulsion thereof is dried up, the poly(silsesquioxane) is obtained in a powdery form. Despite the usefulness of an aqueous emulsion of such a poly(silsesquioxane), no satisfactory method is known in respect of the stability of the emulsion and absence of coarse gelled particles.